This invention relates generally to power monitoring devices, and more particularly to devices for monitoring power carried by a conductor, or power factor.
Numerous devices for measuring power factor have been proposed in the past. For example, Rowell U.S. Pat. No. 2,583,798, Rowell U.S. Pat. No. 1,929,289, Hoare U.S. Pat. No. 2,312,904, and Rich U.S. Pat. No. 2,269,225 all disclose early power measuring methods of apparata. Other devices for measuring voltage, current, or power-related quantities are shown in Arndt U.S. Pat. No. 3,471,784, D'Avino U.S. Pat. No. 3,745,457, Caste Pat. No. 3,584,229, Lieberman U.S. Pat. No. 3,621,392, and Hewlett U.S. Pat. No. 3,562,647.
The Hewlett U.S. Pat. No. 3,562,647 discloses an analog device for measuring power factor which is based on the "balanced bridge" principal, in which the phase angle between voltage and current at zero crossings of these waveforms causes the bridge to become unbalanced as an indication of power factor. However, Hewlett appears to require a direct connection between phase and ground, and is therefore inappropriate for field use.
Another approach is provided by either Liebermann U.S. Pat. No. 3,621,392 or Caste U.S. Pat. No. 3,584,299. Each of these patents discloses a power factor meter in which no conductive connection is made to the conductor under test. For example, Liebermann U.S. Pat. No. 3,621,392 discloses a device which can be used to measure power, power factor, current or voltage without a direct conductance connection. However, these devices require contact with or close proximity to the operator, such that the path of the voltage sensing is from the conductor to the meter and then to ground via either capacitance or conductance through the operator. Thus these devices are limited to use with lines of workable voltage.
Thus a need has existed for a device which accurately detects power factor in high voltage power lines, but which also isolates the operator from the detecting circuit.